feat(agents): align duplicate create errors

internal/handler/api/v1/agent_handler_test.go

@@ -190,10 +190,14 @@ func (s *AgentHandlerTestSuite) TestCreateAgentDuplicate() {
 	s.handler.Create(c)
 	assert.Equal(s.T(), http.StatusOK, w.Code)
 
-	// Try creating again — should conflict
+	// Try creating again — Chatwoot renders ActiveRecord::RecordInvalid as 422.
 	w2, c2 := s.makeRequest("POST", "/api/v1/accounts/1/agents", req, s.account.ID, s.user.ID)
 	s.handler.Create(c2)
-	assert.Equal(s.T(), http.StatusConflict, w2.Code)
+	assert.Equal(s.T(), http.StatusUnprocessableEntity, w2.Code)
+	var data map[string]interface{}
+	json.Unmarshal(w2.Body.Bytes(), &data)
+	assert.Equal(s.T(), "User has already been taken", data["message"])
+	assert.Equal(s.T(), []interface{}{"user_id"}, data["attributes"])
 }
 
 func (s *AgentHandlerTestSuite) TestCreateAgentValidation() {